Preheat the oven to 180°C/gas mark 4. Grease a 20cm-round springform tin and line with baking parchment.
Put the butter, flour and sugar in a food processor and whizz until it resembles breadcrumbs. Pat together to form a ball, then press the dough evenly into the bottom of the tin. Prick all over with a fork, then bake for 18-20 minutes, until just dry to the touch. Allow to cool completely.
Put the chocolate and butter in a bowl and melt over a pan of simmering water, making sure the bowl doesn't touch the water.
Tip the egg whites into a large bowl and whisk until stiff peaks form. In another bowl, whisk the egg yolks with the sugar until pale. Add the chocolate mixture to the egg yolks, then carefully fold in the egg whites until all the mixture is combined.
Pour into the cake tin and bake for 15-20 minutes. Allow to cool, then dust with cocoa powder before serving.